1. Field of the Invention
This invention relates to a platen roller, more particularly to a platen roller to be used in a printing device such as a typewriter, etc.
2. Related Background Art
Printing device such as a typewriter, etc., conventionally has a platen roller which acts as the base plate during printing. The platen roller is ordinarily columnar with a rubber material or resin material 2 wound around a core metal portion 1 as shown in FIG. 1, and characters of the daisy wheel 5 are printed on the paper for printing 3, by pressing of the daisy wheel 5 struck by the printing hammer 6 at the portion where the ink ribbon 4 is contacted on the paper for printing 3 around the platen roller as shown in FIG. 2.
The present inventors have confirmed by experiments that the performance of printing on the paper for printing as obtained above is influenced by the shock load when the daisy wheel 5 is pressed against the paper. The shock load is determined by the hardness of the rubber material, and the strength of the force, speed, etc., of the hammer which strikes the daisy wheel. If the shock load is deficient, inferior printing such as lacking of a part of letter, indistinct outline of letter, etc., will be generated.
Generation of such inferior printing is a serious problem in typewriter of the prior art, and is frequently generated generally when the environment temperature is higher. This inferior printing occurs not only in typewriters of the manual type wherein the shock load becomes irregular, but also in electronic typewriters in which a machine applies constant shock force on a daisy wheel, and particularly the phenomenon of further frequent occurrence is observed when the temperature of the platen roller is elevated by continuous printing for a long time.
Whereas, the present inventor has found that the printing performance is influenced not only by shock load, but also by shock attenuating ability. Printing performance can be improved by improving the shock attenuating ability as represented by resilient elasticity (making smaller resilient elasticity). Also in the case of a rubber material, the shock attenuating ability was found to become greater at a temperature around the glass transition point.